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Structural evolution of a La2Ti2O7 ferroelectric compound possessing perovskite‐type slabs at elevated temperatures was investigated using the single‐crystal X‐ray diffraction technique. The monoclinic low‐temperature phase (L) transformed into the orthorhombic high‐temperature phase (H) via an incommensurately modulated phase (IC) between ∼989 and ∼1080 K. The L–IC transition was considered to be of the first order, with the L+IC two‐phase co‐existing region between ∼989 and ∼1027 K. The structure of IC was determined from the (3+1)‐dimensional superspace representation with a modulation vector q = αao (α ≃ 0.49), where ao is the a‐axis vector of the basic cell. The structural modulation originated from the variation of the tilt angle of the TiO6 octahedra in the perovskite‐type slab in association with small positional displacements of La atoms. The IC–H transition took place at ∼1080 K and was close to the second order. During the IC–H transition, nanoscale flat plate domains having either a cell twin of the L‐type structural modules or a cell twin of the alternating H‐ and L‐type structural modules began to appear in the approximant structure of IC. The thickness of the flat plate domains then grew rapidly along the modulation vector in proportion to (½ − α)−1 as α approached ½ with decreasing temperature. In the two‐phase L+IC co‐existing region, the IC phase consisting of the two types of cell twins was gradually replaced with the low‐temperature monoclinic phase L, which is not cell twinned but rather twinned macroscopically by the L‐type structural modules. Crystals of La2Ti2O7 undergo successive phase transitions between the low‐temperature monoclinic phase and the high‐temperature orthorhombic phase with an intermediate incommensurately modulated phase at around 1000 K. [ABSTRACT FROM AUTHOR]

Abstract: The structural evolution of lanthanum aluminum trioxide (LaAlO3) crystals grown by the Czochralski (CZ) method has been investigated by the in situ single-crystal X-ray diffraction method in the temperature range between 296 and 1195K. The square of the AlO6 octahedral rotation angle in the rhombohedral structure decreased linearly near the transition temperature, suggesting that a second-order phase transition occurs at circa 839K. Changes in the volumetric thermal expansion coefficient show a significant discontinuity at the transition point, supporting an experimental evidence by dilatometry in the literature. [Copyright &y& Elsevier]

Abstract: Changes in structure and dielectric properties at elevated temperatures have been investigated on single-crystals of sodium potassium niobate, Na0.5K0.5NbO3, grown by the flux method. Single-crystal X-ray diffraction studies revealed that the crystals underwent orthorhombic–tetragonal and tetragonal–cubic phase transitions at 465 and 671K during heating and 446 and 666K during cooling, respectively. Both transitions were accompanied by volumetric discontinuities of collapse upon heating and expansion upon cooling, suggesting that the transitions were of the first order. The coordination numbers of an Nb showed a decreasing tendency with decreasing temperature, i.e., 6 in cubic, 5+1 in tetragonal and 4+2 in orthorhombic. An Na atom occupied a slightly different position from the K atom in 12-fold coordination, resulting in fewer coordination numbers of 8+4 in cubic and tetragonal and 7+5 in orthorhombic. The spontaneous polarisation (P s) estimated from the atom positions and formal charges were approximately 0.29Cm−2 in orthorhombic and 0.18Cm−2 in tetragonal. The contribution of the alkaline oxide components to P s was estimated to be approximately 15% in both ferroelectric forms. The temperature-induced transitions were also confirmed through the dielectric constant and dielectric loss at various frequencies and the differential scanning calorimetry. [ABSTRACT FROM AUTHOR]

Since Boeke's finding of a reversible phase transition of calcite (calcium carbonate, CaCO3) at elevated temperatures [Boeke, H. E. (1912). Neues Jahrb. Mineral. 1, 91–121], and following W. L. Bragg's determination of the structure of the room-temperature Phase I [Bragg, W. L. (1914). Proc. R. Soc. Lond. A 89, 468–489.], the high-temperature Phase V of calcite has been an enduring mystery. Here, we summarize a paper on the structure of Phase V [Ishizawa, N., Setoguchi, H. and Yanagisawa, K. (2013). Sci. Rep. 3, 2832], as well as the intermediate Phase IV which exists between Phases I and V, and add new aspects. An in situ single-crystal X-ray diffraction study revealed that the I–IV and IV–V transitions occurred reversibly at approximately 985 and 1240 K, respectively, in a carbon dioxide atmosphere. Phase V was stable only over a narrow temperature range between 1240 and 1275 K. The crystal decomposed immediately at temperatures above 1275 K, leaving a nanoporous calcium oxide reaction product which retained the shape of the parent calcite crystal. The I–IV transition can be described as an orientational order/disorder transition of the carbonate group, occurring within the same space group $R\bar 3c$. In Phase V, the oxygen sublattice is melted. The joint-probability density function obtained from the anharmonic atomic displacement parameters of the oxygen atoms revealed that the oxygen triangles of the carbonate group in Phase V do not sit still at specified Wyckoff positions in the space group $R\bar 3m$, but are instead distributed with equal probability along the undulated circular orbital about the central carbon. The carbonate group in Phase V is no longer flat on the basal plane when the oxygen triangle comes to troughs or peaks in the undulated orbital, but is instead deformed like an umbrella. Assuming that the oxygen triangle migrates about carbon, the carbonate group should repeat the umbrella inversion in Phase V as a function of time. Finally, possible thermal decomposition mechanisms of calcite are briefly discussed. [ABSTRACT FROM AUTHOR]